As is known to those skilled in the art, a linearizer is a device that makes it possible to compensate for the gain compression (“AM/AM” for “Amplitude-Amplitude Modulation conversion”) of a power amplifier (for example of the HPA (for “High Power Amplifier”) type) and the variation of its insertion phase as a function of the incident power (“AM/PM” for “Amplitude-Phase Modulation conversion”). The power amplifiers commonly used in certain fields, such as for example the space field, are either travelling wave tube amplifiers (or TWTAs) or solid state power amplifiers (or SSPAs).
The linearization principle generally used in satellite payloads is predistortion. A predistortion linearizer is a device that is placed upstream of the input of a power amplifier. It must have a gain and phase response as a function of its output power which is the inverse of that of the power amplifier (as a function of its input power) to which it must be coupled, firstly in terms of variation direction (gain expansion instead of gain compression), secondly in terms of variation amplitude (for example +7 dB of gain expansion and +40 degrees of phase expansion in the presence of an amplifier having a saturation compression of −7 dB and of −40 degrees), and thirdly in terms of characteristic form (also called “shape”) so as to “closely follow” the whole characteristic of the amplifier and therefore to linearize all its range to take account of the variations of input power and of envelope of the payload signal (for the adjustment of the linearizer, the user reasons with a single carrier signal (CW) but the payload signal carries a modulation and most usually has a non-constant envelope).
It is possible to adjust the amplitude of the gain and phase expansions of a predistortion linearizer so that its response is matched to a power amplifier to be linearized. Unfortunately, the compression and expansion shapes of the linearizer and of the associated amplifier are not identical in most cases and therefore the linearization is imperfect. For example, if the linearizer has a slow gain expansion (that is to say close to a straight line) then the compression of the amplifier is relatively severe (that is to say close to an exponential function), the adjustment of the linearizer, that corresponds to a total compensation to saturation of the amplifier, leads to a high backward overcompensation for the input powers that are lower than the saturation, which generally degrades the linearity over the whole of the power range of the linearized amplifier. In this case, the optimal adjustment is obtained by compensating for not all of the compression of the amplifier so as not to carry out too much backward overcompensation.
Most predistortion linearizers use non-linear elements such as Schottky diodes. The non-linear behaviour in transmission of the linearizer as a function of its input power is then based on a mechanism called self-biasing of the diode in the presence of a high power level RF signal. The evolution of the bias point as a function of the RF power will in effect induce a particular behaviour of the diode in terms of impedance presented to the incident RF signal. Two main types of biasing of the diode may lead to non-linear self-biasing behaviours that differ depending on whether the diode is supplied by a voltage generator through a high value or low value resistor mounted in series.
In the case of biasing through a high value resistor (R), when the power of the input RF signal increases, the current (ID) that supplies the diode increases and hence the voltage VD at the terminals of this diode reduces (VD=VLIN−RID, VLIN being the supply voltage of the linearizer). Consequently, the current that passes through the diode is substantially constant.
In the case of biasing through a low value resistor (R), when the power of the input RF signal increases, the current that passes through the diode increases because the voltage VD at the terminals of this diode is substantially constant (VD ≈VLIN).
The diode is then used as an impedance that varies as a function of the input RF power in a linearization circuit responsible for converting this impedance variation into a variation of the transmission parameters of the device of which it forms part. The diode(s) may therefore be used in reflection as a load of a hybrid coupler, or in transmission on a propagation line, or else in series or parallel configuration, for example.
Many devices using the conventional self-biasing non-linear mechanism (described above) have been proposed, they are for example described in patent documents U.S. Pat. Nos. 4,992,754, 4,068,186, FR 2 719 954, FR 2 791 197 and FR 2 833 431.
Unfortunately, this conventional self-biasing non-linear mechanism invariably leads to only one type of curve of the gain and phase expansions of the linearizer and therefore does not make it possible to control the shape (form) of the expansions.